Combined the effect is stronger than each on its own. Caffeine takes ~15 minutes to work. Sadly no source handy but i once read that naps are most effective below 15 minutes. Above it is possible to enter deeper sleep states and wake up worse than before.
So when people talk about napping like this, do they ever actually "fall asleep"? I cannot possibly imagine falling asleep in 15m in the middle of my work day or whatever.
I don't fall asleep. But just lying in bed, eyes closed, sleepmask on and low Binaural Beats on the ears helps me when i feel a slump. When i set a timer to 12 minutes i usually feel the urge to get up after roughly ten minutes.
